fix(boards2): change default permissions to enforce single user roles

[jeronimoalbi]

The permissions implementation allows multiple roles per user because custom implementations might need to have more roles within their boards, which might make sense to the realm that implements the permissions.

Boards2 in the other hand defines a fixed set of mutually exclusive roles, owner, admin, moderator and guest, so it must ensure users don't have more than one role.

Without this change it would be possible to assign, for example, the roles owner and admin to the same user, which is fine but semantically doesn't make sense.

[Gno2D2]

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